The present invention generally relates to an apparatus for holding wafer cassettes in a cassette tub during a chemical mechanical polishing (CMP) process and more particularly, relates to a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade equipped with at least one cassette tub pin assembly.
Apparatus for polishing thin, flat semi-conductor wafers is well not in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e, larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al3O2)may also be used in the slurry composition.
When a wafer surface is planarized by a CMP process, the wafer may have to be transferred between various CMP process stations before the planarization is completed. During the transfer of the wafer between various CMP stations, the wafers are stored in a wafer storage cassette in a vertical position and the whole cassette is positioned in a water tank such that the wafers are immersed in water to prevent the slurry solution left on the wafer surface from drying or solidifying. This is an important process step since if the wafers are not stored immersed in water, the solidified particles on the wafer surface becomes a major contamination source and cause serious scratches on the wafer surface during a subsequent CMP operation.
A cassette tub is normally used for holding at least one wafer cassette, i.e. for holding four wafer cassettes, in a tilted position for accessing by a robot blade, as shown in FIGS. 1 and 2. A wafer cassette 10 is positioned in a cassette tub 12 at a tilt angle of about 5.5xc2x0 as measured from the horizontal plane, i.e. from the bottom wall 14 of the tub. A robot 16 that operates a robot blade 18 is used to pick-up wafers 20 that are stored in the wafer cassette 10. The wafers 20 are positioned in slots (not shown) formed on the interior sidewall of the wafer cassette 10. For instance, the robot blade 18, shown in FIG. 2, is in a pickup position when the robot lowers the blade.
In a conventional chemical mechanical polishing apparatus, a cassette tub pin is used to fix the wafer cassette positioned inside the tub. In the original design provided by the machine manufacturer, only one single pin is used for inserting into a cassette tub hole. The single pin is supposed to hold the wafer cassette in position during the step when the robot picks up a wafer from the cassette. However, the single cassette tub pin can easily fall off the cassette tub hole and thus causing the cassette position to shift away from its supposed position, i.e. at a 5.5xc2x0 angle. When the shifting occurs, the robot blade 18, when lowered to pick up a wafer as shown in FIG. 1, may collide with the wafer 20 and thus either causing a wafer breakage or a serious scratch on the wafer. It is therefore an important step in the robot blade pick-up process that a wafer cassette must be positioned precisely at its supposed angle inside the tub for preventing damages to wafers.
It is therefore an object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade wherein the wafer cassette is immersed in water.
It is a further object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade wherein the at least one wafer cassette is held at an angle between about 3xc2x0 and about 7xc2x0 when measured from a horizontal plane.
It is another further object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade wherein at least one cassette tub pin assembly is used for fixing the position of the at least one wafer cassette.
It is still another object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade wherein at least one cassette tub pin assembly is utilized for engaging a planar base portion of a wafer cassette.
It is yet another object of the present invention to provide a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade wherein at least one cassette tub pin assembly is used to fix the position of the at least one wafer cassette by attaching to two mounting rails by screws.
In accordance with the present invention, a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade is disclosed.
In a preferred embodiment, a cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade is provided which includes a tub body of generally rectangular shape with an open top for holding a quantity of water therein, the tub body has two opposing longitudinal sidewalls; a support structure affixed to a process machine onto which the tub body is situated equipped with two mounting rails along the longitudinal sidewalls each having mounting holes for receiving at least one cassette tub pin assembly; at least one wafer cassette that has a planar base portion for engaging the at least one cassette tub pin assembly such that the at least one wafer cassette is maintained at a tilted position of between about 3xc2x0 and about 7xc2x0 when measured from a horizontal plane; and at least one cassette tub pin assembly each having a straight section for engaging the planar base portion of a wafer cassette and two 90xc2x0 bent sections each with a screw hole at a tip portion, each screw hole for engaging one of the mounting holes provided in one of the mounting rails by a screw.
In the cassette tub for holding at least one wafer cassette in a tilted position for accessing by a robot blade, the at least one cassette tub pin assembly is fabricated of stainless steel, while the tub body is fabricated of a chemical resistant, non-particle generating material, or the tub body is fabricated of teflon. The support structure may be fabricated of stainless steel. The planar base portion may be equipped with engagement means for engaging the cassette tub pin assembly, or the planar base portion may be equipped with a recessed slot for engaging the cassette tub pin assembly. The tub may hold four wafer cassettes. The two mounting rails on the support structure may be fabricated of teflon. The screw used to engage the screw hole on the cassette tub pin assembly to the mounting hole on the mounting rail may be fabricated of stainless steel.